This invention relates to fuel cell electrode catalysts comprising alternating platinum-containing layers and layers containing suboxides of a second metal that display an early onset of CO oxidation. Preferably the early onset is manifest as a pre-peak, in addition to the normal Pt CO oxidation peak, in CO oxidation cyclic voltametry. Methods of manufacturing the subject catalysts are also provided. The fuel cell catalysts of the present invention show improved reformate tolerance, and in particular improved CO tolerance.
The fuel stream feeding a hydrogen fuel cell may be supplied by reformation of hydrocarbons. However, the reformate stream will typically contain constituents including CO, CO2, N2 and CH3 in addition to hydrogen. Typical fuel cell catalysts are susceptible to carbon monoxide poisoning, which causes significant loss of power and durability. U.S. Pat No. 4,910,099 discloses one method of improving CO tolerance in fuel cells. In this method, O2 or air is xe2x80x9cbledxe2x80x9d to the anode side of the cell, where it reacts with any CO present in the fuel gas stream to form CO2. However, this method reduces cell efficiency.
U.S. Pat. No. 5,183,713 concerns a fuel cell catalyst with improved CO tolerance comprised of platinum alloyed with 2-10% tantalum. The metals are not alternately layered and suboxides are not taught.
U.S. Pat. No. 5,523,177 concerns a direct-methanol fuel cell including a partially reduced metal oxide anode porous electrode having an oxide content of between 5 wt % and 20 wt %. The exemplary catalyst is composed of an alloy of platinum and ruthenium. This reference does not teach an alternately layered catalyst. The reference indicates that xe2x80x9cpartial reductionxe2x80x9d proceeds by reduction of PtOx, without significant reduction of RuOx. (U.S. Pat. No. 5,523,177 at col. 4, ln. 62-col. 5, ln. 4). In addition, the reference teaches that reduction of RuOx is not desirable. (Id.) The reference does not teach that the catalyst disclosed therein has any favorable CO tolerance characteristics in a hydrogen/air or reformate/air fuel cell.
U.S. Pat. No. 6,040,077 concerns an alternately layered catalyst of Pt and Ru, including Pt and Ru oxides and suboxides.
A seminal review of anode catalyst materials for CO oxidation (Ross, xe2x80x9cThe Science of Electrocatalysis on Biometallic Surfacesxe2x80x9d, Lawerence Berkeley National Laboratories Report, LBNL-40486) cites the following important criteria for the selection of catalyst materials: the material has to be a Pt alloy, the material alloyed to Pt must not be an oxide or readily form an oxide.
Briefly, the present invention provides fuel cell electrode catalysts comprising alternating platinum-containing layers and layers containing suboxides of a second metal, where the catalyst demonstrates an early onset of CO oxidation. Preferably the early onset appears as a shoulder in CO oxidation cyclic voltametry and more preferably as a pre-peak.
In another aspect, the present invention provides fuel cell electrode catalysts comprising alternating platinum-containing layers and layers containing suboxides of a second metal selected from the group consisting of Group IIIb metals, Group IVb metals, Group Vb metals, Group VIb metals and Group VIIb metals. Preferably the second metal is selected from the group consisting of Ti, Ta, W and Mo.
In another aspect, the present invention provides methods of making such catalysts. In particular, the present invention provides methods of making such catalysts by alternate deposition of platinum and second metals in the presence of substoichiometric amounts of gaseous oxygen.
What has not been described in the art, and is provided by the present invention, is a CO tolerant fuel cell catalyst of the present composition or the method of it""s manufacture provided herein.
In this application:
xe2x80x9csuboxide xe2x80x9d means a composition MOx of a metal M having one or more chemical oxidation states MOn where n is one or more positive rational numbers (typically a ratio of small positive integers), wherein x is not equal to any n and wherein x is less than the greatest n; and
xe2x80x9csubstitutedxe2x80x9d means substituted by conventional substituents which do not interfere with the desired product, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
xe2x80x9cpeakxe2x80x9d means a local maximum value;
a xe2x80x9cpre-peakxe2x80x9d, means, in regard to CO oxidation cyclic voltametry, a peak in addition to and appearing at a lower potential than a Pt CO oxidation peak;
xe2x80x9cshoulderxe2x80x9d means a local maximum in the first derivative of a value;
xe2x80x9cearly onset of CO oxidationxe2x80x9d means CO oxidation occurring at a potential lower than that of the Pt CO oxidation peak. The Pt CO oxidation peak typically occurs at around 400 mV, referenced to a saturated calomel electrode, but may be shifted as low as 200 mV by the presence of other metals, e.g. Ru. Early onset of CO oxidation may be demonstrated by CO oxidation cyclic voltametry, where it is indicated by a rise in current reflecting the oxidation of CO. Preferable conditions for performing cyclic voltametry are 80xc2x0 C. temperature, ambient pressure and 25 mV/sec scan rate. More specifically, early onset may be demonstrated by comparison of the subject CO oxidation cyclic voltametry curve to a curve obtained for a Pt catalyst. The y-axis of the subject curve (representing electrical current) is normalized to the Pt curve by multiplying the values of the subject curve by a scaling factor such that the two curves have the same value at the Pt oxide reduction peak (label (1) in FIG. 3). The curves are not normalized in the x-axis (representing electrical potential). xe2x80x9cEarly onsetxe2x80x9d is defined with regard to the region of the subject curve between the point of H2 evolution and a point 140 millivolts more positive than the reference electrode (saturated calomel electrode (SCE)), preferably in the region between H2 evolution and 110 millivolts above SCE, more preferably in the region between H2 evolution and 80 millivolts above SCE, and most preferably in the region between H2 evolution and 50 millivolts above SCE. xe2x80x9cEarly onsetxe2x80x9d is seen where a point in that region of the subject curve has a positive slope or, more preferably, demonstrates at least 10% greater capacitive (double layer) current than the Pt curve.
It is an advantage of the present invention to provide fuel cell catalysts demonstrating improved CO tolerance.